One of the most promising types of optical coupling into/out of “thin” optical waveguides formed within the surface layer of a silicon-on-insulator (SOI) structure has been defined as an “inverse taper”, or “nanotaper”. An inverse taper is generally defined as a terminating portion of a core of a high index contrast waveguide that is used to effectuate optical coupling between a fiber (or other type of optical transmitting device) and a thin waveguide. In a typical device construction, the lateral dimension of the portion of the nanotaper proximate to the core of the waveguide approximately matches the width of the core. The lateral dimension of the nanotaper decreases monotonically along the direction of light propagation until it reaches a small value associated with a ‘tip” (i.e., that portion of the nanotaper distal from the core of the waveguide). The tip portion represents the point at which light first enters the high index contrast waveguide for an “entry” nanotaper, or the point at which light first exits the waveguide for an “exit” nanotaper.
Consequently, the ability to align an associated optical fiber (or an optical fiber with associated coupling elements, such as a lens) with such a small-dimensioned coupling arrangement has proved to be problematic. For the most part, “active” alignment techniques have been employed, where an optical signal is passed through an optical fiber and into the tip of a nanotapered waveguide structure. The optical coupling efficiency (measured as a function of the optical power received at a given point along the waveguide) is used as a calibration signal, where the position of the fiber endface with respect to the tip of the nanotaper is manipulated until maximum coupling efficiency is achieved.
While this “active” method has been found to provide the desired result, it would be preferable for such alignment to be achieved in a “passive” manner; that is, without the need to illuminate the fiber, measure signals and perform adjustments in real time. Thus, a need remains in the art for a passive arrangement for providing coupling between an optical fiber and a nanotapered coupling waveguide as utilized within an SOI-based optical structure.